Reflex klystron oscillator tube



March 4, 1958 c. E. RICH 2,825,844-

REFLEX KLYSTRON OSCILLATOR TUBE Filed Dec. 2. 1953 VENTORS IN CHARLES E.R/Cf/ DAV/D E KENYON A ORNEY United REFLEX KLYSTRGN OSCILLATOR TUBE Charles E. Rich, Hempstead, and David E. Kenyon,

Huntington, N. Y., assignors to Sperry Rand Corporation, a corporation of Delaware Application December 2, 1953, Serial No. 395,748

12 Claims. (Cl. 3155.22)

This invention relates to ultra high frequency generators, and more particularly, is concerned with an improved reflex klystron oscillator.

Frequency modulation of reflex klystrons by varying the reflector voltage in response to the modulation signal is well known. However, it has been found that reflex tubes of usual construction using an ion focused beam exhibit spurious effects in their frequency modulation characteristic. In particular, it has been found that at certain modulation frequencies there may be a shift in the basic carrier frequency which is often accompanied by a large drop in the reflector electrode current. For example, Where frequency modulation over a band of the order of a megacycle or more is employed, frequency shifts as great as megacycles have been observed, with substantial shifts in the carrier being observed in a range from a few kilocycles to several megacycles. These spurious effects result in various degrees of distortion in the modulation and hence in the information transmitted, which seriously limits the usefulness of the reflex klystron for frequency modulation communication.

It is the general object of the present invention to avoid the foregoing and other difliculties of and objections to the prior art practices by the provision of an improved reflex klystron especially suitable for use in frequency modulation systems.

Another object of this invention is to provide a reflex klystron which may be frequency modulated over several megacycles with a minimum of distortion due to frequency shift in the basic carrier frequency.

These and other objects of the invention which will become apparent as the description proceeds are achieved by the provision of a reflex klystron of otherwise conventional design which includes a drift tube in the form of a cylindrical nickel sleeve extending be tween the cathode at one end and the resonator gap at the other end, the sleeve lying along the axis of the electron stream and having a plurality of radial vanes extending inwardly from the inner wall of the sleeve :1 distance not so great as to intercept the forward electron beam. The vanes are also made of nickel and the sleeve and vanes may be carbonized to minimize secondary electron emission.

For better understanding of the invention reference should be had to the accompanying drawing, in which:

Fig. l is a sectional view of a reflex klystron embodying the present invention; and

Fig. 2 is an enlarged sectional view taken on the line 22 of Fig. 1 showing the drift tube.

Referring to Fig. l as illustrated, there is shown a reflex klystron of generally conventional design in which a resonant cavity 10 is part of the vacuum envelope. The cavity 1% is formed by an outer cylindrical wall 12 and a solid conductive end wall 14. The opposite end of the cavity includes a flexible portion 16 mounted around its outer periphery in an annular ring 18 secured to the cylindrical wall 12.

2,825,844 iatented Mar. 4, 1958 An electron gun 20 is positioned'within a cylindrical shell which forms part of the vacuum envelope of the tube and is secured to the lower end wall 14 of the cavity 10. The gun 20 and cylindrical shell 22 are attached to a base 24 which provides electrical connections to the cathode, focusing electrode, and the heater (not shown). The electron gun 20 directs a stream of electrons along the axis of the tube through the cavity 10.

A cylindrical tube 26 projects into the cavity and shields the electron stream from the existing electric fields thus providing a substantially field free space through which the stream of electrons passes. The upper end of the tube 26 is provided with a grid 28 which together with a grid 30 in the upper end wall of the cavity 10 forms a resonator gap in which interaction between the electron beam and the electromagnetic field Within the cavity 10 takes place according to conventional reflex klystron operation for velocity modulation of the beam. Positioned at the lower end of the tube 26 is a smoother grid 31.

A reflector electrode 32 is positioned along the path of the electron stream for reflecting the electrons back through the resonator gap for transfer of energy to the cavity 10. An electrical connection to the reflector electrode 32 is provided through a cap 34 which is supported by an insulator 36 from the metallic wall 38 forming the upper part of the vacuum envelope of the klystron.

Mechanical tuning of the klystron is achieved by means of adjusting screws 40 extending between the annular plate 18 and a second annular plate 42 secured to the wall 38 of the upper part of the klystron. The screws 40 react against springs, such as indicated at 44, adjustment of the screws 40 varying the space between the plates 18 and 42 and thereby varying the spacing between the grids 28 and 30. Spring pressure is varied by means of studs 45 mounted on the plate 18 and extending up through the center of the springs, and adjusting nuts 47 threaded on the studs as shown.

An output signal is derived from the cavity 10 by means of a coaxial line coupler indicated generally at 46, the inner conductor 48 of which extends as a probe within the cavity 10.

The reflex klystron as thus far described, when employed in C. W. operation with frequency modulation, is subject to the spurious effects as mentioned above, namely, at certain discrete frequencies of modulation, there is a sharp shift in the basic carrier frequency which is frequently accompanied by a drop in reflector electrode current and a drop in the FM output. These spurious effects have been substantially overcome by providing, in accordance with the present invention, a plurality of conductive fins or vanes 50, preferably made of nickel, around the inner wall of a nickel cylindrical sleeve 52 pressed into the tube 26, as seen in Fig. 2. These vanes are preferably made in the form of channel members which are spot welded to the inner wall of the sleeve 52.

The vanes thus formed project toward the region of the electron stream but are designed so as not to actually intercept the incident beam but rather to define a cylindrical passage through the vanes of suflicient diameter to permit unimpaired passage of the electron stream from the cathode through the tube 26. Also to prevent interception of the electron stream by the vanes, they are made so as not to extend the full length of the tube 26, but are terminated some distance away from the smoother grid 31 in the region where the electron stream is considerably spread out as it emerges from the gun 20. It

has been found that the provision of the vanes within the tube 26 practically eliminates the spurious effects above described in an ion focused frequency modulated reflex klystron.

The importance and effect ofthe vanesrin suppressing the spurious effects associated with frequency modulation operation of a reflex kylstron may be best understood by considering the theory as presently developed on the cause of the carrier shift.

The electron stream from the cathode is directed through the tube 26 toward the resonator gap. In the region of the tube 26 there is ion focusing of the beam due to the presence of trapped ions within the region between the ends of the tube. However, the electrons reflected by the reflecting electrode, by the time they again pass through the resonato-r'gap, are fairly well scattered and strike the wall of the tube 26 causing secondary electrons to be emitted. These secondary electrons have no noticeable detrimental eflect on the'kylstron operation so long as the klystron is unmodulated.

There exists within the region of the drift tube a potential drop between the electron beam and the Walls of the tube 26. This potential drop is such that a condition is established in which plasma oscillation may be set up in the ions within the tube, that is, if the ion gas is excited into plasma oscillation, this potential drop together with the modulated returning beam tends to maintain the oscillation. The natural frequency of this oscillation of the ion plasma is within the normal modulation frequency range of operation of the reflex klystron.

When the reflector voltage is varied during modulation, the returning electrons strike the wall in such manner that the production of secondaries at any one point is also modulated, thus exciting the plasma oscillation. This oscillation of the ion plasma causes an enlargement of the beam diameter and subsequent shift in frequency of the carrier. Since the power output and frequency of a klystron are dependent on beam diameter, the spreading of the beam in the resonator gap at the modulation frequencies where plasma oscillation is excited gives rise to the described spurious effects in the klystron operation.

Provision of the radial vanes within the tube 26 acts to prevent plasma oscillation in two ways. First, by their geometry they tend to trap a large portion of the secondary electrons produced in the tube 26 and so prevent their entering the ion plasma. Second, the radial fins alter the potential variation particularly in the region near the walls of the tube 26 and thereby remove the condition for maintaining plasma oscillation in the ion gas.

Since to some degree the behavior of the plasma oscillation is dependent on the secondary emission characteristic of the tube wall, altering the surface has a definite effect on the character of the oscillation. It has therefore been'found advantageous to treat the exposed surfaces of the sleeve 52 and vanes 50, as by carbonizing them, to provide a surface having a reduced secondary emission coeflicient to thereby limit the production of secondary electrons.

From the above description it will be seen that the various objects of the invention have been achieved by the provision of an improved reflex klystron tube using ion focusing of the beam. The reflex klystron tube may be frequency modulated by changing the reflector electrode voltage, the modulation'frequency being variable over a video frequency'band with negligible shift in the carrier frequency. This improved performance is achieved by novel means including a plurality of radially projecting fins in the ion focusing region, the fins modifying the potential gradient between the electron beam and the wall of the tube and also trapping secondary electrons emitted by the wall 'of the tube by the scattered reflected electrons.

Since many changes could be made in the above constr'uc'tion'and many apparently widely different embodimerits of thisinvention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electron discharge tube comprising means for producing an electron stream directed along an axis, means positioned along said axis for reflecting the electron stream back along said axis, a cylindrical conductive sleeve having its axis of revolution aligned with the axis of the electron stream, the sleeve being positioned between the electron stream producing means and the reflecting means, first grid across one end of the sleeve, a second grid across the other end of the sleeve, a plurality of conductive fins projecting substantially radially and inwardly from the inner surface of the sleeve, the inner longitudinal edges of the fins being spaced from the axis of the electron stream so as to permit passage of the forward electron stream through the sleeve without interception v by the fins, the fins and the sleeve having carbonized surfaces, and a single cavity resonator positioned along the stream for interaction therewith at a point intermediate the reflector means and the end of the conductive sleeve.

2. An electron discharge tube comprising means for producing an electron stream directed along an axis, means positioned along said axis for reflecting the electron stream along said axis, a cylindrical conductive sleeve having its axis of revolution aligned with the axis of the electron stream, the sleeve being positioned between the electron stream producing means and the reflecting means, a first grid across one end of the sleeve, a second grid across the other end of the sleeve, and a plurality of conductive fins projecting substantially radially and inwardly from the inner surface of the sleeve, the inner longitudinal edges of the fins being spaced from the axis of the electron stream so as to permit passage of the forward electron stream through the sleeve without interception by the fins, the fins and the sleeve having carbonized surface.

3. An electron discharge tube comprising means for producing an electron stream directed along an axis, means positioned along said axis for reflecting the electron stream back along said axis, a cylindrical'conductive sleeve having its axis of revolution aligned with the axis of the electron stream, the sleeve being positioned between the electron stream producing means and the reflecting means, and a plurality of conductive fins projecting substantially radially and inwardly from the inner surface of the sleeve, the inner longitudinal edges of the fins being spaced from the axis of the electron stream so as to permit passage of the forward electron stream through the sleeve without interception by the fins, the fins and the sleeve having carbonized surfaces.

4. High frequency electron beam apparatus comprising tubular conductive means defining a substantially field free space, means aligned with said conductive means at one end thereof for projecting an electron-beam having a predetermined boundary through said space, means for reflecting said beam back into said conductive means, and means including a plurality of conductive members associated with said conductive means and projecting substantially radially inwardly into said space along a substantial portion of the-path of said beam within said space for trapping secondary electrons released by the impingement of electrons of said reflected beam upon said conductive means and members, said conductive members being spaced from said beam boundary for precluding interception of the beam electrons contained by said boundary.

5. High frequency electron beam apparatus comprising conductive means defining a substantially field free space, means defining a velocity modulation gap at One end of said conductive means, means aligned with said conductive means at-the other'end thereof for projecting an electron beam having a predetermined boundary through said space and across said gap, means for reflecting said beam back across said gap and into said conasaaaas ductive means, and means within said conductive means in spaced relationship with said beam boundary for trapping secondary electrons released by impingement of the electrons of said reflected beam upon said conductive means.

6. In an ion-focusing reflex klystron having a conductive tube defining a substantially field free space between the cathode and reflecting electrode of the klystron, a plurality of radially projecting conductive fins secured to the inner surface of the conductive tube, the fins extending longitudinally over a substantial portion of the length of the conductive tube and defining a cylindrical passage of a diameter larger than the diameter of the electron beam.

7. A reflex klystron comprising cathode means for producing a beam of electrons having a predetermined boundary directed along an axis, a reflector electrode positioned along the axis and spaced from the cathode means for intercepting and reflecting the beam back along the axis, a single cavity resonator positioned along the axis and having a velocity modulation gap for interaction with the electron beam at a point intermediate the cathode means and reflector electrode, and means defining a substantially field free space for the electron beam along the axis between the cathode means and the velocity modulation gap of said cavity resonator, said last-named means including conductive means defining a plurality of elongated passages parallel with the axis of the beam for trapping secondary electrons produced by interception of the reflected electrons from the reflector electrode by the field free space defining means, said conductive means being spaced from said predetermined beam boundary for precluding interception of beam electrons contained by said boundary.

8. A reflex klystron comprising cathode means for producing a beam of electrons having a predetermined boundary directed along an axis, a reflector electrode positioned along the axis and spaced from the cathode means for intercepting and reflecting the beam back along the axis, a single cavity resonator positioned along the axis for interaction with the electron beam at a point intermediate the cathode means and reflector electrode, and means defining a substantially field free space for the electron beam along the axis between the cathode means and the cavity resonator, said last-named means including means along a major portion of the length of said field free space for trapping secondary electrons produced by interception of the reflected electrons from the reflector electrode by the field free space defining means.

9. An electron discharge tube comprising means for producing an electron stream directed along an axis, means positioned along said axis for reflecting the electron stream back along said axis, a cylindrical conductive sleeve having its axis of revolution aligned with the axis of the electron stream, the sleeve being positioned between the electron stream producing means and the re fleeting means, and a plurality of conductive fins projectsnbstantially radially and inwardly from the inner surface of the sleeve, the inner longitudinal edges of the fins being spaced from the axis of the electron stream so as to permit passage of the forward electron stream through the sleeve without interception by the fins.

10. A reflex klystron, comprising means including a cathode for producing and directing an electron beam having a predetermined boundary along a predetermined axis, a cavity resonator having a reentrant electron permeable metallic tube portion followed by a velocity modulating gap in axial alignment with said beam, the inner diameter of said tube portion being larger than said beam boundary, means positioned along said axis beyond said gap for reflecting the electrons of said beam back along said axis past said gap and into said tube portion, and means along said tube portion between the inner wall thereof and the boundary of said beam for substantially isolating said beam in its path toward said reflecting means from secondary electrons within said tube portion.

11. A reflex klystron as set forth in claim 10, wherein said last named means comprises a plurality of conductive members supported within said reentrant portion and projecting into the space between said beam and tube portion for trapping secondary electrons.

12. A reflex klystron as set forth in claim 11, further including a pair of electron permeable grid members at opposite ends of said tube portion, said grid members enhancing ion trapping within said tube for beam focussing.

References Qited in the file of this patent UNITED STATES PATENTS 2,497,564 Sorg Feb. 14, 1950 2,581,408 Hamilton Jan. 8, 1952 2,608,670 Ginzton Aug. 26, 1952 

